Exhaust gas purifying apparatus such as catalyst are provided in exhaust gas pipes of internal combustion engines in order to purify exhaust gas. Various technical ideas for supplying secondary air to upstream sides of these exhaust gas purifying apparatus have been proposed in order to improve purification efficiencies of the exhaust gas purifying apparatus. If secondary air is not normally supplied, then purification efficiencies of exhaust gas purifying apparatus are lowered, which may deteriorate exhaust emission. As a result, various technical ideas capable of detecting abnormal statuses of secondary air supply systems have also been proposed.
For instance, in JP-9-21312A (corresponding to U.S. Pat. No. 5,852,929), while the pressure sensor is installed in the secondary air passage, the abnormal condition of the secondary air supply system is detected based upon the detection value of the pressure sensor under such a condition that the secondary air pump is operated. In JP-2003-83048A (corresponding to US-2003-0061805A1), the malfunction modes of the respectively structural components of the secondary air supply system are detected based upon combinations of pressure behavior patterns when the secondary air is supplied, and also, when supplying of the secondary air is stopped.
In order to properly manage exhaust gas amounts of exhaust emissions, there are necessities to detect secondary airflow rates. However, in such secondary air supply systems as described in the above-described publications, it is practically difficult to detect the secondary airflow rate in high precision. In other words, in the conventional secondary air supply systems, the pressure (namely, secondary air supply pressure) at the air output ports of the secondary air pump is basically detected, and then, such a calculation method for calculating the secondary airflow by employing the detected secondary air suppress pressure may be conceived. However, when this calculation method is conducted, there is a problem that the calculation precision as to the secondary airflow rate is deteriorated due to tolerance (fluctuations of performance etc.) of products. When the secondary air pump is constituted by DC motor, or the like, there is certain product tolerance (fluctuations of performance etc.) in the secondary air pump. In addition, pipe pressure loss may be produced in second air pipe through which secondary air flows. The pressure sensor also owns individual differences and tolerance. These factors may cause another problem that the calculation precision of the secondary airflow rate is deteriorated.